1. Field of the Invention
The invention is directed to a main element usable as a rotor or stator for an electrical machine, especially for a small DC motor.
2. Description of the Prior Art
The magnetically conductive bodies of stators or rotors in an electrical machine are increasingly being made from SMC (soft magnetic powder iron composite) material and particularly for small DC motors are replacing the laminated iron bodies or lamination packets, since from a production standpoint they are very much easier to manufacture. SMC material moreover offers additional advantages in comparison to stator or rotor bodies of sheet-metal lamination packets, such as reducing iron losses at high frequencies in motors with a high rpm or a large number of poles, improved thermal characteristics because of the use of thin tooth insulators, since the teeth can be pressed without sharp corners and edges, and high flexibility in terms of design of the motor. SMC material is typically pressed into the desired shape of the stator or rotor using a pressing tool and is then heat-treated at a relatively low temperature, for example of up to 500° C. In the pressing, a relatively high density of the material, which comes as close as possible to that of iron (7.8 g/cm3), must be attained. Such a density can be attained only whenever the aspect ratio of the compact is as high as possible. An aspect ratio of 1:15 has been established as a limit value. The term “aspect ratio” is understood to mean the ratio between the smallest dimension of the cross section, extending transversely to the pressing direction, of the compact to the length of the compact in the pressing direction.
To maintain this kind of aspect ratio, in a known multipole stator of SMC material for an internal rotor machine (International Patent Disclosure WO 99/50949), it is known for the outer short-circuit ring, also called a short-circuit yoke, to be composed of a number of annular segments in the circumferential direction that correspond to the number of teeth of the stator. Each annular segment of the short-circuit ring integrally has one stator tooth with a tooth neck and a tooth shoe that is the boundary of the tooth neck on its end remote from the short-circuit ring. Each annular segment with a tooth neck and tooth shoe is produced from SMC material by pressing and heat treatment. The tooth necks are rounded on their axial ends or have an oval profile, so that with the elimination of sharp edges at the tooth necks, only a thin insulating layer has to be applied, onto which the annular coil can then be wound. The individual annular coils of the stator winding are wound directly onto the tooth necks using conventional machine winding technology. Once the annular coils have been wound on, the individual annular segments are placed against one another in the circumferential direction and solidly joined together.
In another known stator of SMC material for an internal rotor machine (WO 00/69047), the short-circuit ring on the one hand and the stator teeth with the tooth neck and tooth shoe on the other are produced separately in the desired form from SMC material. Once the prefabricated annular coils are slipped onto the tooth necks, the stator teeth are inserted by positive engagement, with the ends of the tooth necks remote from the tooth shoe, into prepared recesses in the short-circuit ring and secured to it.